1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and more particularly to a capacitor element provided within a semiconductor integrated circuit device.
2. Description of the Prior Art
A semiconductor integrated circuit device often contains some capacitor elements. The so-called "p-n junction capacitor" and "MIS (Metal-Insulator-Semiconductor) capacitor" are employed as such capacitor elements. The p-n junction capacitor utilizes the depletion layer (space charge region) extending from a p-n junction between P and N type semiconductor regions. The MIS capacitor includes an insulator film as a dielectric formed on a semiconductor which serves as a lower electrode, and an upper electrode formed on the insulator film. The MIS capacitor is more advantageous in less parasitic effect and a large capacitance per unit area, as compared with the p-n junction capacitor.
In recent years, the semiconductor integrated circuit device has utilized the selective oxidation technique, whereby a large number of circuit elements are isolated from one another by means of a thick field oxide film formed by the selective oxidation technique. According to the selective oxidation technique, the surface of a semiconductor substrate is selectively oxidized by making use of an oxidation-resist film as a mask to produce a thick field oxide film partly buried in the substrate. A nitride of a semiconductor (for instance, a silicon nitride film) is widely employed as such oxidation-resist film. The relative dielectric constant of the silicon nitride film is about 7, and that of the silicon oxide film is about 3.6. That is, the silicon nitride film has a dielectric constant that is about twice as large as that of the silicon oxide film.
Thus, the MIS capacitor employing the silicon nitride film as a dielectric has a relatively large capacitance. Accordingly, the oxidation-resist film used for the selective oxidation is selectively left, and a MIS capacitor element is formed by making use of this left oxidation-resist film as a dielectric.
However, as is well known, an edge portion of the field oxide film produced by the selective oxidation is formed in a shape of a bird's beak (hence the edge portion being called "bird's beak"), and due to a strain generated during the oxidation process, an edge portion of the oxidation-resist film turns up. As a result, the oxidation-resist film is separated from the field oxide film, and a part of the substrate may be exposed. Consequently, a conductor layer formed on the oxidation-resist film as an upper electrode of the MIS capacitor contacts with the exposed part of the substrate at the periphery of the oxidation-resist film. Since the substrate portion under the oxidation-resist film acts as a lower electrode of the MIS capacitor, the above-mentioned contact between the conductor layer and the part of the substrate causes a short circuit between the upper and lower electrodes of the MIS capacitor, so that the structure no more operates as a capacitor element. It has been proposed to form a very thin oxide film of a semiconductor between the oxidation-resist film and the substrate to enhance the adhesion therebetween. Even by such measure, the thin oxide film cracks, so that the upper electrode contact with the substrate. Even if the short circuit does not occur, the withstand voltage of the capacitor element is greatly lowered.